In general, an air drying system removes moisture included in the air. The air drying system is used in a variety of fields throughout the industries, such as various automation equipments, a semiconductor manufacturing process, a production line in a chemical process causing chemical reactions by contact with moisture, etc., all of which require dry air.
The air drying system is divided into a cooling air drying system and an adsorption-type air drying system. The cooling air drying system decreases the temperature of compressed air by using a cooling compressor and condenses the moisture included in the compressed air, thereby removing the moisture in the air. The adsorption-type air drying system adsorbs the moisture included in the air by using an adsorbent, a dehumidifying agent, and a moisture-absorbent.
The adsorption-type air drying system is increasingly used which has an excellent energy efficiency, is easy to install, and is economical in maintenance.
Generally, the adsorption-type air drying system is divided into a heat adsorption-type air drying system and a non-heat adsorption-type air drying system in accordance with a regeneration method of the adsorbent. The heat adsorption-type air drying system regenerates the adsorbent by using a predetermined heat source. The non-heat adsorption-type air drying system regenerates the adsorbent only by air for regeneration.
Also, the heat adsorption-type air drying system is divided into a circulation heat adsorption-type air drying system and a non-circulation heat adsorption-type air drying system. The circulation heat adsorption-type air drying system regenerates the adsorbent by causing a compressor to circulate the compressed air. The non-circulation heat adsorption-type air drying system regenerates the adsorbent by inhaling outside air. Here, the used air is discharged to the outside.
Various examples of the adsorption-type air drying system are disclosed in Korean Patent Nos. 10-0701218, 10-0750190, 10-0793980, and 10-0976553.
For example, FIGS. 1 to 3 show a conventional representative adsorption-type air drying system.
As shown in FIG. 1, the adsorption-type air drying system includes two towers 100a and 100b in which the adsorbent is built; a generation line 110 for generating dry air; a regeneration line 120 for regenerating the tower; a cooling line 130 for cooling the tower; valve devices 140a and 140b which are installed in generation line 110, the regeneration line 120 and the cooling line 130 and convert the air flow; coolers 150a and 150b and separators 160a and 160b which are installed in the generation line 110 and the regeneration line 120 respectively and cool the air and separate the moisture; a heater 170 which is installed in the regeneration line 120 and heats the air; and a plurality of the valves 180a to 180e which are installed in the lines respectively and control the air.
Here, an undescribed reference number 210 represents the compressor which provides the compressed air.
A process in which a generation operation and regeneration heating operation/regeneration cooling operation are formed in the adsorption-type air drying system which is configured as described above will be described as follows.
The generation operation is to generate the dry air by adsorbing the moisture in the air. The regeneration heating operation is to heat and provide the air to the tower and to separate the moisture adsorbed in the adsorbent in the tower, so that the adsorbent is regenerated. The regeneration cooling operation is to cool the tower which has been overheated after the regeneration.
The generation operation and regeneration heating operation/regeneration cooling operation are simultaneously performed in the two towers. For example, the regeneration heating operation/regeneration cooling operation are sequentially performed in a second tower during the generation operation in a first tower. Reversely, the regeneration heating operation/regeneration cooling operation are sequentially performed in the first tower during the generation operation in the second tower.
As shown in FIG. 2, first, the compressed air which is supplied by the compressor 210 is branched off at a first branch point 190a, and then about 70% of the compressed air flows through the generation line 110 and the remaining about 30% of the compressed air flows through the regeneration line 120.
Here, the flow rate distribution of the compressed air at the first branch point 190a can be controlled by a flow rate control valve or an orifice, etc.
Next, the air flowing through the generation line 110 is cooled by the first cooler 150a and the moisture in the air is separated through the first separator 160a. Then, the air passes through a junction 200 and is introduced into the first tower 100a through the first valve device 140a. 
The moisture in the air introduced into the first tower 100a is removed in the first tower 100a. Then, the dry air where the moisture has been removed passes through the second valve device 140b, and then is transferred to the process.
Simultaneously with this, 30% of the compressed air branched off at the first branch point 190a flows through the regeneration line 120 at a temperature of 80 to 170° C. The air flowing through the regeneration line 120 is heated at a predetermined temperature required for the regeneration, for example, 120 to 250° C. by the heater 170, and then is introduced into the second tower 100b through the second valve device 140b. 
Then, the adsorbent is regenerated by the high temperature air introduced into the second tower 100b. 
At this time, the air flows from the top to the bottom of the second tower 100b (top-down approach).
Continuously, the regenerated and heated air passes sequentially through the first valve device 140a, the second cooler 150b and the second separator 160b and is joined at the junction 200 to the air flowing through the generation line 110, and then is transferred to the first tower 100a. Then, a drying process is performed on the air.
Meanwhile, after the adsorbent is regenerated, a process of cooling the overheated tower is immediately performed after the regeneration.
For this, as shown in FIG. 3, while the compressed air which is supplied by the compressor 210 flows through the generation line 110, the compressed air is cooled by the first cooler 150a and the moisture in the compressed air is separated by the first separator 160a. Then, the compressed air is branched off at a second branch point 190b and about 70% of the air decompressed by the flow rate control valve 200a continues to flow through the generation line 110 and the remaining about 30% of the air flows through the cooling line 130.
In FIG. 3, a reference numeral 180b represents the flow rate control valve installed in the cooling line 130.
Next, the air flowing through the generation line 110 passes through the junction 200 and is introduced into the first tower 100a through the first valve device 140a. The moisture in the air introduced into the first tower 100a is removed in the first tower 100a, and then the dry air where the moisture has been removed passes through the second valve device 140b and is transferred to the process.
Simultaneously with this, the air flowing through the cooling line 130 passes through the first valve device 140a and is introduced into the second tower 100b. The second tower 100b is cooled by the air introduced into the second tower 100b. 
At this time, the air flows from the bottom to the top of the second tower 100b (bottom-up approach).
Next, the cooled air passes sequentially through the second valve device 140b, the second cooler 150b and the second separator 160b and is joined at the junction 200 to the air flowing through the generation line 110, and then is transferred to the first tower 100a. Then, the drying process is performed on the air.
Here, the foregoing has described an example of the operation state in which the dry air is generated in the first tower, and at the same time, the regeneration heating operation and regeneration cooling operation are performed in the second tower. It can be also considered that the operation is also performed by interchanging the functions of the first and second towers through the opening and closing state of the valves, flow path change, and air flow path change due to them.
That is, through the tower change, the dry air is generated in the second tower, and at the same time, the regeneration heating operation/regeneration cooling operation are performed in the first tower. The generation operation and the regeneration heating operation/regeneration cooling operation are interchanged and alternately performed in the two towers.
However, the above-described conventional adsorption-type air drying system has the following problems.
The air flow is formed in the top-down approach in the regeneration heating operation, and the air flow is formed in the bottom-up approach in the regeneration cooling operation. Therefore, the flow path of the entire system is configured in a complicated manner and the number of components for controlling the air flow, such as a valve, etc., is increased.
Also, too many components to be controlled, such as a valve, etc., require intricate control and the operating pressure condition of the system is limited (the operating pressure of a typical non-purge operation system is 6 to 7 bar). Therefore, the conventional adsorption-type air drying system is difficult to use under various pressure conditions and is difficult to smoothly operate when the pressure change of the system occurs.